Injection-moulded article comprising a field of hooks obtained by moulding

ABSTRACT

One-piece article obtained by injection molding, with at least one injection point, said article comprising a main body ( 20 ), having a body volume defined by the space within a surface forming an outer envelope, and at least one hook ( 21 ), preferably a field of hooks, obtained from the same molding of the main body of the molded article, the hook or each hook having a stem of longitudinal axis and a catching part projecting laterally from the stem, characterized in that the volume of the hooks is between 100,000 and 100,000,000 times smaller than the volume of the body, and in that the width, or smallest transverse dimension, of the stems, measured transversely to the longitudinal axis, is smaller than the thickness of the main body, measured along the longitudinal axis of the stems.

The present invention relates to an article which is in a single piece,obtained by injection moulding at one injection point at least,consisting of a main body with a body volume which is defined by thespace within an outer surface, the moulded article being for example aninner door panel of a motor vehicle.

The invention also relates to a device forming an injection mould whichis designed to form by injection moulding a moulded article of thistype, as well as a block which forms a moulding insert which is designedto be inserted in a mould of the aforementioned type.

Moulded articles of this type are already known in the prior art.

The intention is to add to the outer surface of the article at least onehook, and preferably a field of hooks:

-   1) which is obtained from the same moulding as the main body, and is    integral with the latter;-   2) which has a volume of hooks which is substantially smaller than    the volume of the body, i.e. at least a hundred times smaller,    preferably at least a thousand times smaller, and for example    between 100,000 and 100,000,000 times smaller than the volume of the    body; and-   3) the hooks of which are thin, i.e. which have their smallest    transverse dimension at the level of their base which is smaller    than the thickness of the article at the level of the base, measured    along the straight line which is perpendicular to the surface.

The or each hook is designed to form the male part of a self-grippingdevice with hooks and loops, or hooks in hooks, in order to co-operatein particular for example with loops or hooks of another article inorder to secure the moulded article to this other article.

In the present application, hook means an element comprising a stem anda catching part which projects laterally from the stem, in particular aharpoon, a double hook, a mushroom and the like.

Hitherto in the prior art, in particular as described inEP-B1-0,577,697, it has already been possible by moulding a part in asingle mould to provide plates with hooks comprising a plate with asmall thickness and thick hooks, which in particular have a widthgreater than the thickness of the plate. However, by means of the methoddescribed, it is not possible to obtain hooks which have a very smallwidth dimension of their stem.

In EP-B1-0,577,697 an injection mould is used in order to form theobject moulded (an orthopaedic knee piece) on the outer surface of whichthere is formed by moulding in the same mould a field of hooks which arethen designed to co-operate with loops. Conventionally, part of theinner surface of the mould comprises a field of cavities with a formwhich is approximately complementary to the hooks to be produced. Thisfield of cavities is disposed in a position which is suitable for thehooks to be formed in the precise location where they are to be formedon the final moulded article. There is then injection into the mould ofthe moulding material, which in particular is thermoplastic, and fillsthe inner space of the mould and also the moulding cavities of thehooks, in order thus to obtain the moulded article comprising a field ofhooks produced integrally with part of the outer surface of the article.

However, this mould and this method do not make it possible tomanufacture the moulded article with hooks with a small volume and/or asmall width.

In fact, the disadvantage is that the pressure of the injection of thethermoplastic material is not sufficient for the latter to penetrateefficiently into the cavities for formation of the hooks, which cavitieshave a small dimension relative to the volume of the final object.

There is already known from the prior art, and in particular from theU.S. Pat. No. 6,224,364, a system based on over-moulding which makes itpossible to improve the quality of filling of the cavities of the fieldof cavities by the thermoplastic material, and to obtain hooks with asmaller height and width than the thickness of the article at the levelof the hooks. However, the technology which is proposed in this priordocument has several disadvantages. Firstly, the final article obtainedis not in a single piece, but is in two pieces which are over-moulded onone another, and are separated by an interface in the material itself ofthe article, which separates the hooks from most of the rest of thearticle. However, this interface involves fragility of the article whichis detrimental to its service life. In fact, this interface canultimately constitute an area of reduced resistance to the traction ofthe hooks, and give rise to rapid deterioration of the moulded article,and in particular to detachment of the area of the field of hooks fromthe moulded article, thus making the latter unusable in co-operationwith another article with loops or hooks to which it is to be secured.Furthermore, the device which forms a mould has a complicated design,which makes it essential to provide a plurality of openings forinjection of the material, and in particular an opening for injectionopposite the field of cavities for formation of the hooks, and to have apart which is mobile relative to the rest of the mould, which causesdifficulties both of design and use, in particular in terms of sealingand long-term reliability. In addition, once the mould has beenconstructed, it is possible to modify only with difficulty the positionof this field of cavities anywhere in the mould, without manufacturingthe mould entirely once more. The method thus described also has thedisadvantage of being in two stages, which extends the manufacturingtime of the final moulded article.

According to the invention, it has been possible to obtain for the firsttime an article which is in a single piece obtained by injectionmoulding at one injection point at least, consisting of a main body witha body volume which is defined by the space within a surface which formsan outer envelope, and at least one hook, and preferably a field ofhooks, which is obtained from the same moulding as the main body of themoulded article, the hook or each hook having a stem with a longitudinalaxis, and a catching part which projects laterally from the stem,characterised in that:

-   -   the volume of the hooks is substantially smaller than the volume        of the body, i.e. at least a hundred times smaller, preferably        at least a thousand times smaller, and for example between        100,000 and 100,000,000 times smaller than the volume of the        body; and    -   the width, or smallest transverse dimension of the stem,        measured transversely to the longitudinal axis, is smaller than        the thickness of the main body, measured along the longitudinal        axis of the stem.

Preferably, the largest height dimension of the hook or each hook,measured along the longitudinal axis, is smaller than the thickness ofthe main body measured along the longitudinal axis of the stem.

Preferably, the field of hooks covers only a small part of the outersurface, which in particular is strictly less than half this outersurface, and is preferably less than 30%.

Preferably, the width of the base of the stem is less than 0.29 mm, andin particular it is between 0.05 and 0.15 mm.

In order to obtain a moulded article of this type, according to anotheraspect of the invention, it is possible to use a device which forms amould, in order to form a moulded article by injection of a mouldingmaterial, which in particular is thermoplastic, comprising an innerclosed surface which defines a main moulding chamber, at least oneopening for the introduction of the moulding material being formed inorder to be able to introduce the moulding material by injection intothe moulding chamber, at least one cavity with a form complementary toat least one hook, and preferably a field of cavities, opening onto apart of the inner surface, the said one part of the inner surface beingsurrounded by an edge surface which does not comprise a cavity,characterised in that means are provided for thrusting at least part ofthe edge surface and an opposite part of the inner surface towards oneanother, the arrangement being such that the thrusting by the thrustmeans is limited, such that the two surfaces which are thrust towardsone another cannot come into complete contact with one another, and inparticular there is always a gap between the two surfaces.

The present invention overcomes the disadvantages of the devicesaccording to the prior art by proposing a device which forms a mouldwith a simple design, which in particular may need only a single openingfor the introduction of the thermoplastic material, and makes itpossible to obtain a final moulded article comprising hooks which areintegral with the moulded article, whilst being able to position thefield of hooks on substantially any location of the outer surface of themoulded article, and which makes it possible to obtain a moulded articleand a field of hooks which are truly integral, and in particular doesnot comprise an interface between the main part of the article and thefield of hooks, which have a small dimension in relation to the body ofthe article.

In the known systems of the type described in U.S. Pat. No. 6,224,364,the thermoplastic material which is designed to form the hooks is notassisted in penetrating into the cavities in a manner which is moreforceful than in the prior systems in which the cavities for formationof the hooks are positioned such that the latter are formed at the endof filling of the moulding cavity. In particular, the inventors of thepresent invention have realised that a phenomenon occurs which can beclassified as hesitation, which consists of the material “hesitating” topenetrate into the cavities for formation of hooks, and a type of skinthen forms which blocks the opening of the cavity and prevents thematerial from penetrating into it, such that, when the moulded articleis extracted from the mould, it does not have hooks, or hooks aremissing from the field in relation to the number of cavities. Accordingto the invention on the other hand, there is improvement of thepenetration of the material into the cavities, for the samethermoplastic material and form of the cavities of the hooks, byassuring that the pressure of introduction of the thermoplastic materialinto the cavities for moulding of the hooks is very great, very quickly,and in particular is faster than in the systems previously known. Thisrapid increase in the pressure has the effect of increasing the speed atwhich the material arrives in front of the openings of the cavities,thus tending to prevent the formation of the skin as occurs in the priorart, and therefore makes it possible to obtain hooks with a relativelysmall volume.

According to a preferred embodiment, the main moulding chamber has aform which is substantially complementary to the article to be moulded.

Preferably, the gap has a very small dimension, and in particular isbetween 0.01 and 0.1 mm.

According to the invention, this gap is measured in the directionperpendicular to the two surfaces which face the point(s) of the saidone surface part furthest away from the opposite surface.

In particular, this gap or play is very much smaller than the depth ofthe cavities with a form complementary to the hooks, and in particularat least ten times smaller, and preferably at least a hundred timessmaller.

Thus, according to the invention, it is no longer necessary to providean opening which opens into the said opposite surface in order to formthe part with hooks, after most of the moulded article has been formed.In particular, it is possible to introduce the thermoplastic mouldingmaterial via a single opening, and this thermoplastic material will fillall of the main chamber, with the exception of the gap between thesurface with hooks and the opposite surface, the dimension of the gapbeing in fact selected to be so small that the thermoplastic materialcannot penetrate into it unless a pressure corresponding to that whichprevails when substantially all of the chamber is filled with mouldingmaterial is applied to it, and it is only when most (i.e. apart from thegap) of the chamber has been filled, with the pressure of thethermoplastic material increasing, that the latter will be able toinfiltrate into the gap, whilst going against the force created by thethrust means. As soon as the material starts to infiltrate in the gap,the force applied by the material becomes far greater than the thrustforce. The volume above the cavities is then suddenly freed, and thematerial then fills the volume, as well as the cavities which form thehooks, very rapidly, under very a high pressure, in the manner of anexplosion.

Since the moulding is carried out in a single step of introduction orinjection of the thermoplastic material into the mould, there is no needto wait to mould the hooks, as was the case in the prior art, for mostof the thermoplastic material to have cooled, in order, then, to formthe hooks, and consequently no interface is formed in the materialbetween the field of hooks and the rest of the moulded article, and themoulded article and the hooks are formed uniformly in a single piece.One of the particular features of the product obtained is that thematerial penetrates via the entire periphery of the area which isthrust, whilst gathering in a central area according to a pattern whichcan be a point, a line or a star, according to the form of the field ofhooks. Since the material front does not have the time to cool becauseof the speed of the operation, the joining area does not constitute atrue interface (in particular there is no interface visible to the nakedeye) between the hooks and the rest of the moulded article, andconsequently this does not constitute an area of greater fragility ofthe part. In particular, only an analysis of molecular orientation wouldmake it possible to reveal this area.

Preferably, the at least one opening, and in particular the introductionopening(s), in particular for injection of thermoplastic material intothe mould, open(s) into the moulding chamber at a part of the innersurface which does not comprise the said opposite surface.

According to a preferred embodiment, the entire edge area is maintainedspaced from the opposite surface part.

According to a preferred embodiment, the edge area and the inner surfaceare in continuity with one another, and are maintained spaced from theopposite surface.

According to another possible embodiment, the edge area is constitutedby a shoulder of the inner surface part, and only the edge part ismaintained spaced from the opposite surface.

According to another possible embodiment, the edge area comprises acurved part which comes into contact with the opposite surface at itstop, with the rest of the edge area being maintained spaced from theopposite surface.

According to a preferred embodiment, only a part of the edge area ismaintained spaced from the opposite surface.

According to a particularly preferred embodiment, an insert block isprovided which comprises an upper surface in which there is formed acavity or a field of cavities with a form complementary to hooks, whichis disposed in a detachable manner within the moulding cavity, inparticular in a location where the hooks in the final article are to beformed, and the thrust means are integrated in the insert block, withthe upper surface of the insert block constituting the said innersurface part of the mould.

Thus, in order to form the field of hooks in any location in the mouldedarticle, the insert block is placed in the moulding cavity in thelocation where the hooks are to be formed, without being restricted by apre-existing design of the mould.

According to a preferred embodiment of the invention, the thrust meansconsist of at least one spring. It is also possible for the thrust meansto be constituted by a hydraulic or pneumatic jack.

The present invention also relates to an insert block as used in a moulddevice according to the invention.

The present invention also relates to a method for manufacturing bymoulding of a moulded article comprising a hook or a field of hooksobtained integrally with part of its outer surface, which consists oftaking a device which forms a mould according to the invention, andinjecting thermoplastic material via an opening, in a single step ofintroduction, in particular of injection.

For example in the case of a motor vehicle door panel, the ratio may begreater than 10⁵.

Preferably, the hook has a size or height smaller than 0.9 mm, and inparticular between 0.2 mm and 0.7 mm.

In particular, the moulded article comprises on its outer surface atleast one area in the form of a hollow or point, corresponding to thepoint of injection of the material during manufacture by injectionmoulding of the moulded article.

By way of example, a description is now provided of a preferredembodiment of the invention, with reference to the drawings, in which:

FIG. 1 is a view in transverse cross-section of a device which forms amould according to the invention;

FIG. 2 is a view in perspective of the insert block of the device whichforms a mould in FIG. 1;

FIG. 3 is a partially cut-out view of FIG. 2;

FIGS. 4 a, 4 b and 4 c are diagrams explaining the progression of themoulding operation when the device which forms a mould in FIGS. 1 to 3is used;

FIG. 5 is a view substantially equivalent to that in FIG. 3 of anotherpossible embodiment of a moulding insert according to the invention;

FIG. 6 is a diagram representing the curve which gives the injectionpressure of the thermoplastic material as a function of time duringimplementation of the moulding method according to the invention;

FIGS. 7 a, 7 b, 7 c, 7 d, 7 e and 7 f are diagrams describing aplurality of possible arrangements according to the invention of theedge area, of the surface within the cavities and the opposite surface;

FIGS. 8 a and 8 b represent another embodiment of a device according tothe invention, wherein the parts which have the same functions as theembodiments in the other figures have the same numerical references; and

FIG. 9 represents in transverse cross-section the moulded articleobtained at the end of the moulding step in FIG. 4 a.

FIG. 1 represents a mould constituted mainly by two parts 1 and 2, whichare closed on one another in order to define a moulding chamber 3 withthe form of the article to be moulded. An opening 4 for introduction byinjection of the thermoplastic moulding material is formed in the upperpart 1. An insert block 5 is disposed in the moulding chamber 3. Theinsert block 5 can be disposed substantially anywhere in the cavity.However, it is ensured that it is not located opposite the opening 4.

The insert block 5, which can be seen better represented in FIGS. 2 and3, is constituted substantially by a rectangular parallelepiped blockwith an upper surface 6 in which there is formed a field of cavities 12with forms complementary to hooks to be formed on an outer surface ofthe moulded article. The insert block comprises a parallelepiped baseblock 7 and a mobile block 8. The block 7 is designed to be secured(preferably in a detachable manner) relative to the inner surface 9 ofthe chamber 3, whereas the block 8 is mobile relative to the block 7,and thus also relative to the mould. A spring 10 is integrated in theblock 5, and thrusts the mobile block 8 towards a part of the innersurface of the chamber, and in particular towards a part 11 of thissurface which is substantially opposite the upper surface 6 of themobile block 8 which comprises the cavities 12 with forms complementaryto the hooks. The upper surface 6 of the mobile block can be subdividedinto two parts, i.e. a central part 6′ which is delimited by theoutermost hook cavities, and an edge 6″ which surrounds the central part6′

The spring is regulated such that, when unloaded, i.e. without thepressure in particular of the thermoplastic material injected into thecavity, the upper surface 6 is at a short distance from the surface 11,and in particular such that a gap or play of approximately 0.01 to 0.1mm is formed. This play is such that the two surfaces 6 and 11 are notin complete contact with one another, and are however such thatthermoplastic material cannot infiltrate into it until all of thechamber, apart from the gap and the cavities 12, has been completelyfilled. In particular, the edge part or area 6″ without a cavity of theupper surface 6 is maintained spaced from the surface part 11 opposite.

This play represents 1/10 to 1/100 of the depth of the cavities with acomplementary form of the moulds (or of the height of the hooks).

It should be noted that the upper surface 6 of the insert 5 has a smalldimension relative to the inner surface 9, such that the insert istotally surrounded by the remaining volume of the chamber 3.

The use of the device which forms a mould is as follows. Fluidthermoplastic material is injected via the opening 4 in order to form amoulded article with a form which is complementary to the chamber 3.Initially, the thermoplastic material is introduced into the volumewhere the resistance to its progression is the least great, i.e. itfills all of the chamber 3 with the exception of the gap between thesurface 11 and the surface 6, this gap being too small for thethermoplastic material to be able to be introduced into it by itselfwithout pressure, or at a pressure lower than that which exists when theentire chamber 3 is filled. Once substantially all of the chamber 3 hasbeen filled, with the introduction of the thermoplastic materialcontinuing, the pressure of the latter in the volume occupied in thechamber 3 increases, and the thermoplastic material, under the effect ofthis pressure, then attacks the gap between the two opposite hooksurfaces (6 and 11), and the pressure becomes such that, suddenly, thematerial is introduced rapidly into this gap and thrusts the insertblock, in particular its mobile part 8, against the force of the spring10. The presence of this spring and the small dimension of the gap meanthat the pressure which is necessary in order for the thermoplasticmaterial to begin to be introduced is strong. As a result, thepenetration of the thermoplastic material into the gap takes placesuddenly, in a manner which is almost identical to an explosion, withgreat force and great kinetic energy, such that the thermoplasticmaterial is introduced efficiently into the cavities 12 which have formscomplementary to the hooks, and the hooks are well “formed”.

Once the hooks are formed, introduction of the thermoplastic material isstopped, it is allowed to cool, and the half-mould 1 is opened inrelation to the half-mould 2 in order to extract from the chamber 3 thefinal moulded article, which comprises on its outer surface a field ofhooks with forms complementary to the moulding cavities 12 which are onthe upper surface 6 of the insert block 5. This field of hooks isintegral with the rest of the moulded article, and, in particular, thereis no interface which forms a separation in the material of the article.

The moulded article which is obtained at the end of the step in FIG. 4Cis represented in FIG. 9. It consists of a main body 20 (without hooks)in the form of a plate, and hooks 21 issuing from the plate, whilstbeing in a single piece and integral with the latter, and in particularobtained from the same mould. In the direction perpendicular to thelower surface of the plate, from which the hooks project, the height hof each hook is smaller than the thickness e of the plate, these twovalues being measured along the axis of the stem of the respective hook,which in this case is also perpendicular to the outer surface of thebody 20. The width of the base of the stem, measured on the planeperpendicular to the axis of the stem, is smaller than the thickness eof the plate. In addition, the field of hooks covers only part of thelower surface of the plate, such that it covers less than 50% of thetotal outer surface of the plate. In this case it covers less than 30%of the plate, and even in reality less than 10%, in the knowledge thatFIG. 9 is a view in cross-section at the level of the hooks, and thefield of hooks, in the direction perpendicular to FIG. 9, extends onlyover a relative dimension in relation to the plate, which is identicalto that over which it extends in the direction represented in thefigure, this being deduced without difficulty from the form of theinsert 5 in FIG. 2, 3 or 5.

According to the embodiment represented in FIG. 5, the edge area 6″ ofthe upper surface is curved. This curvature ensures that it is formed inco-operation with the surface part 11, opposite a type of wedge alongthe peripheral edge, which wedge will make it possible to facilitate theattack of the thermoplastic material, when the latter will be attemptingto introduce itself between the two surfaces 6 and 11, which howevercome into contact with one another at the level of the central area 6′.Nevertheless, the contact between the two surfaces is not complete,since at the level of the peripheral edge area, the two surfaces are notin contact.

According to another embodiment, which is not shown, but is covered bythe present invention, it would also be possible to form the cavities inthe inner surface 9, in particular in the part 11 of the surface, and toobtain the upper surface 6 from the full block (i.e. without cavities).With an embodiment of this type the advantage is admittedly not obtainedof being able to place the field of hooks where required on the finalmoulded article, however, one of the important effects of the inventionis obtained, i.e. the formation of the hooks by means of virtuallyinstantaneous injection.

In the embodiment which is represented schematically in FIG. 7 a, theedge area 6″ is constituted by a shoulder relative to the central area6′, with the thrust means thrusting the surface 6′ into contact with theopposite surface 11, but with the shoulder keeping the edge area 6″spaced from the opposite surface.

In the embodiment which is represented schematically in FIG. 7 b, theedge area 6″ comprises curvature which projects relative to the centralarea 6′, with the thrust means thrusting the top of the curvature of thearea 6″ into contact with the opposite surface 11. However, the slopingpart of the curvature on the outer side of the central area 6′ is keptspaced from the opposite surface, thus creating a wedge which permitsthe attack and entry of the thermoplastic material when the pressurebecomes sufficient to counter the force of the thrust means.

In the embodiment represented schematically in FIG. 7 c, twoprotuberances 30 project from the surface of the central area 6′, withthe thrust means thrusting these protuberances into contact with theopposite surface 11, but with the rest of the surface 6, and inparticular the edge area 6″, being kept spaced from the oppositesurface.

In the embodiment represented schematically in FIG. 7 d, only the edgearea 6″ is thrust by the thrust means, with the central hook area 6′being secured relative to the opposite surface and spaced from thelatter. The thrust means thrust the area 6″ (which surrounds the centralarea 6′) towards the opposite surface, whilst however keeping it spacedfrom this opposite surface. When the pressure of the moulding materialbecomes sufficient to counter the force of the thrust means, the area 6″moves away from the opposite surface, and the material can penetrateinto the central area 6′.

In the embodiments represented schematically in FIG. 7 e or 7 f, theedge area 6″ has a part which is eccentric. The thrust means thrust theupper surface 6 (constituted by the central area 6′ and the eccentricedge area 6″) towards the opposite surface. The thrust is howeverlimited, such that the eccentric part remains spaced by a small gap fromthe opposite surface, with the rest of the surface 6 coming into contactwith the opposite surface.

FIG. 6 shows schematically the form assumed by the curve which gives theinjection pressure of the thermoplastic material, as a function of thetime during the manufacture of a moulded article. In a first interval oftime I, the pressure rises in a regular increasing manner, correspondingto the filling of the volume of the chamber, apart from the gaps and thehook cavities. After the interval I has elapsed, the pressure increasesmore rapidly during the interval II, until it reaches a pressure Pmax.During a very short interval III the filling of the gap and the cavitiestakes place, with the injection pressure decreasing slightly beforeincreasing once more. According to the invention the injection takesplace continuously and in a single step.

FIGS. 8 a and 8 b show another embodiment of a moulding device accordingto the invention.

As can be seen in FIG. 8 a, the insert block 5 consists of twosub-blocks 51 and 52. The upper block 51 defines in its interior, on theside opposite its upper surface 6, a receptacle 53 which is open on thelower side, and receives the sub-block 52. In the sub-block 52 twolateral recesses 54 are formed, which open onto the lateral surfaces ofthe sub-block 52, and into which there are received respectively twosprings 55, each comprising at their free end a lug 56. In the innerlateral walls of the sub-block 51, two notches 57 are formed, with aform which is substantially complementary to the lugs 56.

In the position represented in FIG. 8 a, the upper surface 6 is spacedfrom the surface 11 of the mould. The lugs 56, which are thrust by thesprings 55, project from the recesses 54, and penetrate at least partlyinto the notches 57, such that the two sub-blocks are rendered integralwith one another. Starting from this position in FIG. 8 a, thethermoplastic material is introduced into the mould. When the pressureof the material becomes sufficient, the sub-block 51 is thrustdownwards, whilst thrusting the lugs into the recesses 54, such that thesub-blocks 51 and 52 are released from one another, and go into theposition represented in FIG. 8 b, thus opening up to a large extent thegap between the surfaces 11 and 6, in order to permit filling of thecavities 12. Once the part has cooled, the ejectors 58 thrust the partto the exterior of the mould, and the traction force of the mouldedhooks is sufficient to return the two sub-blocks into the position inFIG. 8 a.

In the present invention, upper surface of the insert block (with thereference 6 in the figures) means all of the points on the outer surfaceof the insert block which are seen when the block is viewed from abovein a vertical direction. The central part (with the reference 6′ in thefigures) also means the upper surface of the part of the central surfaceinto which there open all the cavities which are designed to form hooks,and which is delimited by the points furthest to the exterior of theedges furthest to the exterior of the openings of the cavities which arefurthest to the exterior of all the cavities, and by the straight lineswhich connect these points to one another, such that the closed curvethus obtained includes all the cavities in its interior. Finally, theedge part (with the reference 6″ in the figures) is the part of theupper surface on the exterior of the curve which delimits the centralpart. The edge part surrounds the central part.

The invention claimed is:
 1. A moulded article obtained by injectionmoulding of a material, the article comprising a main body having a bodyvolume which is defined by the space within a main body outer surfaceforming an outer envelope, and a field of hooks formed integrally withthe main body of the moulded article, each hook having a stem with alongitudinal axis and a hooking part which projects laterally from thestem, wherein: the moulded article is made from thermoplastic material;the total volume of the hooks forming the field of hooks is between100,000 and 100,000,000 times smaller than the body volume of the mainbody; the width, or smallest transverse dimension of the stem, measuredtransversely to the longitudinal axis, is smaller than the thickness ofthe main body, measured along the longitudinal axis of the stem; themain body and the field of hooks form a single piece obtained by asingle injection moulding step, the article having no interface in thematerial; and the moulded article further comprises on an outer surfaceat least one area in the form of a hollow or point, corresponding to anopening at a point of injection of the material during manufacture byinjection moulding of the moulded article, where the surface of themoulded article opposed to the field of hooks is devoid of such area. 2.A moulded article according to claim 1, wherein the field of hookscovers strictly less than half the main body outer surface.
 3. A mouldedarticle according to claim 1, wherein the largest height dimension of atleast one hook of the field of hooks, measured along the longitudinalaxis, is smaller than the thickness of the main body measured along thelongitudinal axis of the stem.
 4. A moulded article according to claim1, wherein the width of the base of the stem of at least one hook of thefield of hooks, which is its smallest dimension transversely to thelongitudinal axis, is less than 0.29 mm.
 5. A moulded article accordingto claim 1, wherein at least one hook of the field of hooks has a sizeor height smaller than 0.9 mm.
 6. A moulded article according to claim1, wherein the field of hooks covers less than 30% of the main bodyouter surface.
 7. A moulded article according to claim 1, wherein thewidth of the base of the stem of at least one hook of the field ofhooks, which is its smallest dimension transversely to the longitudinalaxis, is between 0.05 and 0.15 mm.
 8. A moulded article according toclaim 1, wherein at least one hook of the field of hooks has a size orheight between 0.2 mm and 0.7 mm.